1. Field of the Invention
The invention relates to an arrangement for generating a broadband spectrum which can be used in particular as a light source for short coherence interferometry.
2. Description of Related Art
Light sources with a short coherence length and a broadband spectrum are required, for example, in three-dimensional surface shape recording and in optical coherence tomography (OCT). OCT represents a noninvasive imaging method which makes the topography of surfaces and structures in scattering media visible. Light in the near IR is used for the examination of, for example, biological tissue, on account of its greater penetration depth into the tissue.
The measurement principle is based on an optical interferometer which determines the scattering power and depth position of the structures with a high resolution. The resolution of the OCT is dependent, inter alia, on the light source used. In OCT, the photons scattered in the tissue are filtered out on the basis of their interference properties. This requires a light beam with the shortest possible coherence length (but >0) and a broadband spectrum in the near IR. The three-dimensional resolution in the beam direction corresponds to the coherence length of the light used. The greater the coherence length, the greater the volumetric region from which information is backscattered. In modern OCT systems, the resolution is up to 10 micrometers. Resolutions of approx. 10 micrometers can be achieved, for example, using commercially available OCT appliances with superluminescent diodes which emit in the near IR. Although these diodes have a lower light efficiency than comparable laser diodes, their coherence length is short and they therefore allow a good resolution to be achieved in the appliance.
However, for many applications, for example in tumor therapy, an improved resolution at cellular level is required but cannot be achieved using commercially available appliances.
Furthermore, it is known to generate what is known as a supercontinuum by introducing intensive, ultrashort light pulses into a nonlinear optical medium.
In this context, photonic crystal fibers (PCF) are of increasing interest. These fibers comprise microstructured fibers, for example formed from a fiber core and a microstructured fiber cladding with a periodic structure (photonic band gap fiber), as described by J. C. Knight et al. in Optics Letters, Vol. 21, No. 19, P. 15-47 (October 1996), or a nonperiodic structure, as disclosed by U.S. Pat. No. 5,802,236, which surrounds the core and runs along the fiber length. Suitable structuring and formation of the fiber cladding and dimensioning of the fiber core give rise to index conduction of the radiation in the fiber. The radiation can be concentrated with a high intensity in the core by employing an effective refractive index difference between the fiber core and the fiber cladding (5% to 20%). These fibers typically comprise microstructured silicon oxide fibers.
The third-order nonlinear effects (χ(3)) which are essential to the generation of a supercontinuum, such as the self-phase modulation, only occur with short light pulses with a high peak intensity. Investigations carried out by Ranka, Windeler, Stenz in Optics Letters, Vol. 25, No. 25 (2000) have shown that sufficiently high field intensities to activate nonlinear processes in order to generate a supercontinuum in microstructured silicon oxide fibers can be achieved using femtosecond laser pulses.
Since the intensity of the light pulse corresponds to the ratio of pulse power to cross-sectional area of the fiber, and since the pulse power is determined by the ratio of pulse energy to pulse duration, to achieve nonlinear effects it is possible, within the context of what is technically feasible, either to shorten the pulse duration and/or to increase the pulse energy, for example by increasing the repetition rate of a laser, and/or to reduce the cross-sectional area of the fiber core of the fiber.
An output spectrum which covers the visible region and the near IR can be achieved with a core diameter of approx. 2 micrometers in microstructured silicon oxide fibers with an anomalous dispersion, as described for example in U.S. Pat. No. 6,097,870, and with a 100 femtosecond laser pulse from a titanium-sapphire laser (typical pulse energy 1 to 12 nJ, pulse power approx. 8 kW). For propagation of the pulse through the fiber, the geometry of the fiber (core, cladding structure) has to be adapted to the wavelength of the laser pulse, in such a manner that the zero dispersion of the group velocity is approximately at the wavelength of the laser pulse.
Therefore, the resolution of measurement arrangements used for short coherence interferometry could be increased by using Ti-sapphire lasers, but such lasers are large, unwieldy, unstable and expensive and are therefore unsuitable for use in a light source for OCT appliances or for other commercial short coherence measuring appliances.